Monitoring circuit for a door operating D.C. electric motor

ABSTRACT

A monitoring circuit for a door operating D.C. electric motor, the circuit comprises a first means for detecting a back EMF of the motor and producing a load signal indicative of the magnitude of the back EMF, a second means for detecting the time rate of change of the load signal and issuing an obstruction signal if the magnitude of the time rate of change of the load signal exceeds a predetermined value.

TECHNICAL FIELD

This invention relates to apparatus for detecting obstruction of a motordriven door.

The invention was developed for detecting obstruction of a motor drivengarage door during opening and closing operation and will herein bedescribed with reference to that use but it will be understood that theinvention is equally applicable to use for detecting obstructing ofother types of electric motor operated doors.

BACKGROUND OF THE INVENTION

Motor driven garage doors of the upward opening and downward closingtype have been provided with obstruction detection switches operated bymeans of a trip bar at the lower leading edge of the door. The switch isarranged so as to stop or reverse the door if the trip bar strikes anobstruction during descent. While such means are effective they add tothe complexity of the door manufacture and installation and therefore tocost.

It has been practiced to sense the absolute load of the driven systemeither by reaction of the drive, for example as described in Australianpatent specification No. 521,735, or by change in current flow in themotor circuit. Such systems suffer from nuisance trips due to changes inambient and local conditions or are maladjusted or desensitized so thatnuisance trips do not occur but then the system tends to become a safetyhazard. When the door is a rolling door then the motor load changesprogressively and non-linearly between open and closed positions as aresult of friction and making adjustment of trip level even moredifficult. Thus, to date, for rolling doors the only satisfactory methodfor reversing the door when an obstruction is encountered during descenthas been the trip bar previously described.

DISCLOSURE OF THE INVENTION

According to one aspect the invention consists in a monitoring circuitfor a door operating D.C. electric motor, said circuit comprising firstmeans for detecting the back EMF of the motor and producing a loadsignal indicative of the magnitude thereof; and second means fordetecting the time rate of change of the load signal and issuing anobstruction signal if the time rate of change of the load signal exceedsa predetermined value.

For preference, the apparatus monitors the back EMF continuously and issensitive to changes occurring during any interval of predeterminedshort duration.

According to a second aspect the invention consists in a monitoringcircuit according to the first aspect further comprising third means forcomparing said load signal with a reference signal to issue anobstruction signal if said load signal changes by a predeterminedmagnitude.

As is well known the speed of a DC motor reduces with increasing loadand the back Electro Motive Force (EMF) developed by the motor isproportional to motor speed. Thus if the load is increased the motorspeed decreases causing the back EMF to decrease. The present inventionprovides apparatus which produces a signal responsive to the rate ofchange of back EMF of the motor. This may be achieved by sampling theback EMF during one time interval and comparing it with a stored EMFdetected during a previous time interval the time intervals being atpredetermined spacing. In a preferred embodiment a differentiatingcircuit is used to generate a signal the voltage of which isproportional to the change or back EMF of the motor during apredetermined period, that is to say, the rate of change of back EMF. Ifthe differentiated signal voltage exceeds a predetermined voltage thenthe circuit trips and an obstruction signal is issued which may be usedto stop or reverse the motor.

By way of example only in embodiment of the invention will now bedescrioed with reference the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is schematic circuit diagram of a monitoring circuit according tothe present invention.

BEST MODE OF CARRYING OUT THE INVENTION

In the embodiment described a steel roller garage door is driven betweenopen and closed positions by a low impedance 24 voltage DC motorenergised from a 240 volt: 24 volt step down transformer via a full wavebridge rectified unfiltered power supply. The motor is connected to thepower supply via a relay so connected as to enable the polarity ofsupply to the motor to be reversed. By means of the relay the directionof run of the motor and thus the direction in which the door is drivenmay be reversed.

With reference to the schematic circuit shown in FIG. 1, lead 10 isconnected to the positive side of the motor when the door is travellingdownwards. Resistors R1 and R2 provide a high forward impedance whilediode D1 provides a low reverse impedance.

Since the motor supply voltage is unfiltered it will vary between thepeak D.C. supply voltage and zero at twice the frequency of the voltagesupplied to the step down transformer. When connected to the positiveterminal of the D.C. motor the supply voltage therefore varies betweenthe peak DC value and a non-zero positive voltage due to the back EMF ofthe motor. The magnitude of the non-zero positive voltage provides ameasure of back EMF of the motor. Resistors R1 and R2 block charging ofcapacitor C1 by the supply voltage appearing at lead 10 whilst the lowreverse impedance provided by diode D1 allows capacitor C1 to bedischarged to the lower no zero positive voltage reached by the motorsupply. Thus the variations or ripple of the voltage appearing at thepositive terminal of the DC motor which is connected to lead 10 resultin capacitor C1 being charged to a voltage which varies according to theback EMF of the motor. Resistor R3 and capacitor C2 act as a filter tosmooth voltage spikes and noise so that the voltage at circuit junction20 substantially varies only according to the back E.M.F. of the motor.

In the presently described embodiment a door obstruction signal isissued in two circumstances. An obstruction signal is issued if the rateof change of back EMF falls at a rate of change which exceeds apredetermined rate ("dynamic load"). An obstruction signal is alsoissued if the back EMF falls below a predetermined voltage ("staticoverload").

The static overload part of the circuit will be described first. A1 isan operational amplifier functioning as a comparator to compare thevoltage at terminal 5 (varying according to motor speed) with areference voltage at terminal 6 derived via resistor R6. In the presentembodiment the adjustment of sensitivity is obtained by varying theinput impedance via resistor R1 rather than by adjustment of thereference voltage as this provides additional smoothing. If for anyreason the motor slows to such a Point that the voltage at 5 goes lowrelative to the voltage at 6 then the output 7 goes low. When the outputat 7 goes low, input 3 to operational amplifier A2 goes low relative toinput 2. Operational amplifier A2 is arranged as a non-inverting A.C.amplifier by connection of resistors R9 and R10 and capacitor C4.Operational amplifier A2 has no D.C. gain and the D.C. Voltage level ofoutput 1 follows the D.C. voltage level of input 3. Thus when input 3goes low relative to input 2, the output terminal 1 goes low which tripsa switching means (not shown) connected at 11. The switching meanscomprises a Schmitt trigger (not shown) and a reversing relay (notsnown) activated by the Schmitt trigger to cause the door motor toreverse or if preferred may be arranged to cause the door to stop. Bythis means the downwardly moving door is reversed if the door speedfalls below a predetermined absolute speed or is otherwise overloaded.

The "dynamic overload" part of the circuit will now be described.Capacitor C3 and resistors R7 and R4 form a high pass filter ordifferentiating circuit. The differentiating circuit is isolated fromoperational amplifier A1 by diode D2. The voltage signal at circuitjunction 30 therefore varies according to the time rate of change of thevoltage signal at junction 20. If the voltage at 20 decreases (as aresult of decreasing D.C motor speed), the voltage at 30 is decreased byan amount proportional to the time rate of change of voltage at 20. Theconstant of proportionality or differentiation constant is determined bythe component values of the elements forming the differentiatingcircuit.

Therefore, if the voltage at junction 20 falls at a rate greater than apredetermined rate dependent on the values of C3, R7, and R4, the input3 of operational amplifier A2 will go low relative to input 2 causingoutput 1 of A2 to go low. As described above operational amplified A2 isarranged as a non-inverting AC amplifier by connection of resistors R9and R10 and capacitor C4. R9, R10 and C set the A.C. gain and frequencyresponse of operation amplifier A2. These components can also beselected to adjust the time rate of change of input voltage sensitivityof the amplifier A2. In this way the sensitivity to the rate ofdeceleration can further be insured. Output 1 going low trips a Schmitttrigger which causes the door to reverse as described above. Thedescending door is thus caused to stop or reverse if there is a changein motor velocity which exceeds a predetermined time rate of change.

The sensitivity to rate of deceleration may be adjusted by alteration tothe resistance of R7. This has two main effects. Firstly, a change inresistance R7 alters the differentiation constant thus varying themagnitude of the voltage applied at 30 by the rate of charge of voltageat 20. In addition, change in resistance R7 alters the D.C. level ofterminal 3 since R7 and R4 form a potential divider between circuitsupply +V and ground. As the D.C. level of terminal 3 is varied themagnitude of the differentiated signal required to force terminal 3 lowrelative to terminal 2 is altered. In particular, the level ofsensitivity may be decreased by switching resistor R8 into parallel withR7. In the present example an accompanying door controller logic circuit(not shown) is arranged to bypass the circuit of FIG. 1 during the firstsecond when the motor is switched on in the downward direction, that isto say, while the voltage and back EMF are affected by the startingload. After one second the voltage and back EMF have usually stabilizedand the circuit of FIG. 1 is switched in but with resistor R8 remainingopen circuit. After 3.75 seconds resistor R8 is switched from opencircuit to logic high in parallel with resistor R7 by connectingterminal 40 to circuit supply +V. The sensitivity is thereby reduced,that is to say increasing the change in speed required to reduce theoutput of operational amplifier A2 to produce an obstruction signalat 1. When a "door down" command signal is received by the logic circuitthe motor commences operation in a downward direction and after aninitial one second override the door operates with a high level ofsensitivity. Thus in the region near the fully open position in whichrolling type steel doors have a tendency to balloon or unwind internallywhen the door meets an obstruction, the obstruction signal is readilytripped. The high sensitivity also provides very low reversing forcesshould the door strike somebody around the head. However after 3.75seconds when the door is advanced towards fully closed, the sensitivityis reduced so that a steady increase in load for example due to doorfriction will have no influence. However a sudden increase in load dueto an obstruction will trip the dynamic load detector.

Alternatively, the door controller logic circuit can be arranged toswitch in the circuit of FIG. 1 after one second and then progressivelydecrease the sensitivity required to produce an obstruction signal asthe door decends. This can be achieved by replacing R7 with a widelyvariable resistance or by sequentially switching several resistors intoparallel with R7.

Typically, the components used in the circuit of FIG. 1 are as follows:

    ______________________________________                                        R1 - 470Ω    +V = 8 volts                                               R2 - 100kΩ   C1 4.7 μF                                               R3 - 100kΩ   C2 1 μF                                                 R4 - 1MΩ     C3 1 μF                                                 R5 - 100kΩ   C4 1 μF                                                 R6 - 47kΩ    D1 - IN4004                                                R7 - 1MΩ     D2 - IN914                                                 R8 - 1MΩ     A1 - LM358                                                 R9 - 470kΩ   A2 - LM358                                                 R10 - 1MΩ                                                               ______________________________________                                    

It has been found that preferred embodiments of the invention areparticularly sensitive and cause a rolling shutter door to reverse witha very slight but rapid increase in load and yet the system can beapplied to maladjusted doors and to doors of varying size withoutrequiring additional adjustment. The static load sensing device detectsa stall or near stall condition of the motor.

The presently described garage door control systems is provided with aremote operating button which is connected to the logic circuit of thecontroller by means of two wires. The remote button is energized at oneof three levels of current flow by the control logic circuit. When thedoor is in the fully closed position the current level is at its lowest.When the door is not in the closed position tne level is at a highervalue and when the button is depressed and closes the contact a thirdand higher level of current flow is achieved. The button device has alight emitting diode which is switched on when the middle level ofcurrent is flowing. It will not switch on at the lower level of currentwhen the door is in the closed position and is switched off when thebutton is depressed and closes the contact. This device thus providestwo wire remote indication of a condition in which the garage door hasbeen left in a "not closed" condition and operates without regard topolarity or the two wire connection.

As will be apparent to those skilled in the art the inventive concepthereof can be carried into effect by other analog and digital circuitmeans without departing from the teaching hereof and such other meansare deemed within the scope of this disclosure.

We claim:
 1. A monitoring circuit for a door operating D.C. electricmotor, said circuit comprising first means for detecting the back EMF ofthe motor and producing a load signal indicative of the magnitudethereof; and second means for detecting the time rate of change of theload signal and issuing an obstruction signal if the magnitude of thetime rate or change of the load signal exceeds a predetermined value. 2.A monitoring circuit as claimed in claim 1 further comprising thirdmeans for comparing said load signal with a reference signal to issue asecond obstruction signal if said load signal changes by a predeterminedmagnitude.
 3. A monitoring circuit as claimed in claim 1 or 2 whereinthe back EMF is monitored to produce a continuous load signal.
 4. Amonitoring circuit as claimed in claim 1 or claim 2 wherein said firstmeans is connected to a terminal of the DC motor to produce the loadsignal.
 5. A monitoring circuit as claimed in claim 4 wherein the motoris energized by a full wave rectified unfiltered supply.
 6. A monitoringcircuit as claimed in claim 5 wherein said first means has a highforward impedance and a substantially lesser reverse impedance.
 7. Amonitoring circuit as claimed in claim 6 wherein the forward and reverseimpedances are adjustable to vary the magnitude of said load signal. 8.A monitoring circuit as claimed in claim 1 or claim 2 wherein saidsecond means includes a differentiating circuit to detect the magnitudeof the time rate of change of the load signal.
 9. A monitoring circuitas claimed in claim 8 wherein impedance of the differentiating circuitis adjustable to vary the magnitude of the time rate of change of theload signal.
 10. A monitoring circuit as claimed in claim 9 wherein saidsecond means compares a voltage signal generated by the differentiatingcircuit and indicative of the magnitude of the time rate of change ofsaid load signal with a reference voltage signal to produce saidobstruction signal.
 11. A monitoring circuit as claimed in claim 10wherein the voltage signal generated by said differentiating circuit issuperimposed on a D.C. voltage level before comparison with thereference voltage signal, said D.C. voltage level being adjustable todetermine the magnitude of the time rate of change of said load signalrequired to produce said obstruction signal.
 12. A monitoring circuit asclaimed in claim 1 or claim 2 further comprising switching means adaptedto reverse the motor on receipt of at least one of said obstructionsignal and said second obstruction signal.
 13. A monitoring circuit asclaimed in claim 1 or claim 2 further comprising a door controller logiccircuit adapted to prevent issue of any obstruction signal whilst saidmotor is under starting load after having been energized.
 14. Amonitoring circuit as claimed in claim 13 wherein said door controllerlogic circuit is adapted to reduce the sensitivity of said monitoringcircuit after the elapse of a predetermined time interval from saidmotor being energized in a door closing direction.
 15. A monitoringcircuit as claimed in claim 1 or claim 2 wherein said door is a garagedoor of the upward opening and downward closing type.